1. Field of the Invention
The present invention is an improvement on the stereo enhancement system of my prior application, Ser. No. 929,452, filed Nov. 12, 1986, and enables my prior invention to be used with a monaural input signal. The present invention relates to an improved feature of the generation of synthetic stereo signals from a monaural signal and more particularly relates to synthetic generation of sum and difference stereo signals of a type which provide useful stereo information to a stereo enhancement system.
2. Description of Related Art
In many stereo sound systems the circuits merely amplify right and left channel signals and feed these to loudspeakers. In my above-identified co-pending application, stereo signals such as sum and difference signals are processed to provide image enhanced stereo output signals to a stereo speaker system. In these systems and other stereo systems it is necessary that a stereo input be provided if a stereo output is to be produced. Generally such a stereo input is available either in the form of left and right stereo input signals, or, as in some broadcast systems, in the form of the sum (L+R) of left and right stereo signals and the difference (L-R) between such left and right stereo signals. In a common type of stereo signal broadcast system, left and right stereo signals are combined at the broadcast station before transmission. A sum signal (L+R) is modulated upon a main carrier, and a difference signal (L-R) is modulated upon a higher frequency sub-carrier. Generally the sub-carrier is weaker than the main carrier, and transmission of the stereo signals is frequently along multiple paths due to the bouncing of the FM transmission between or among buildings or other obstacles. This causes the difference signal transmitted on the weaker sub-carrier to be considerably weaker at a receiving station, varying in intensity, and fading in and out according to location of the receiver. When such a receiver is mounted in a moving vehicle, it may occur that the difference signal received is so weak as to be substantially useless. For such conditions some receivers are arranged to ignore the weak difference signal and to receive, process and transduce through its loudspeakers solely a monaural signal in the form of the sum (L+R).
Therefore, where the difference signal is too weak or absent, the listener will only be able to receive and hear a monaural sound. This is so even if the receiver should include effective and sophisticated stereo image enhancement circuitry, such as described in detail in my above identified co-pending application. Only in the presence of a stereo input will certain image processing circuits, such as the stereo enhancement system of my prior application, be able to perform the desired enhancement
In other situations only a monaural signal is produced, but stereo sound is desired. For example, when playing a monaural record in a stereo playback system, it would be desirable to provide both left and right stereo signals to the system amplifier, whether or not any enhancement circuitry is employed. So, too, when a vocalist or individual instrumentalist provides sound to only a single microphone, it may be desired to provide stereo sound from the single monaural signal.
Therefore it is desirable to enable a receiver, a playback system, a recording system, or any other sound system, to provide stereo sound even though but a single signal, a monaural signal, is available.
Accordingly, it is an object of the present invention to provide a stereo image enhancement system capable of use with a monaural input.